The invention relates to a simple, inexpensive electronic circuit for providing temperature compensation to an electronic device such as a semiconductor integrated circuit (ICE).
A common practice to implement an electronic system having desired characteristics is to assemble various discrete components including semiconductors and the like each having a specific functionality required for the electronic device. However, it is often the case that assembling such components fails to provide the very desired functionality under certain conditions.
For example, some of the components poses a problem that they lose their anticipated characteristics at a lower and/or higher temperature than room temperature if they have desired characteristics at room temperature. Conventionally, when such problem arises, a different semiconductor circuit must be sought for or the function block associated with the semiconductor must be modified to circumvent problem. In cases where a solution for such problem is not found, a compromise is made to limit the use range of the electronic device. It is obvious, however, that these measures are not true solutions to the problem.
As an example, take a conventional voltage controlled oscillator VCO as shown in FIG. 10(a). In FIG. 10(a), the voltage controlled oscillator VCO has an external terminal for connection with an external resistor R, and an input terminal IN which is supplied with a variable voltage indicative of signal S (said voltage hereinafter referred to as signal voltage S) from a variable voltage source. The oscillator is intended to generate an output of frequency f at its output terminal OUT. The output frequency f is presumably proportional to the signal voltage S as shown in FIG. 10(b), so that the signal voltage S is chosen to provide a required frequency f.
In actuality, however, the frequency f of the voltage controlled oscillator VCO varies with temperature, if the signal voltage S is kept constant. This is because not only the external resistor R but also various components of the voltage controlled oscillator VCO have temperature characteristics which cause the frequency f to change with ambient temperature as shown by a broken curve or a dotted curve in FIG. 10(c) as opposed to the ideal behavior of an oscillator having no temperature dependence as indicated by a solid curve in FIG. 10(c).
FIG. 11(a) shows a circuit diagram of a data lock voltage controlled oscillator VCO that follows the data of a digital video cassette recorder (DVC). It is shown that the voltage controlled oscillator VCO is provided at a control terminal C thereof with a control voltage which determines the free running or natural oscillation frequency of the oscillator VCO. The control voltage for the free running oscillation is obtained by dividing the voltage difference between the supply voltage VCC and the voltage of a power supply B by the resistors (1.8 kilo-Ohms and 15 kilo-Ohms in the example shown herein). A desired frequency f of 41.85 MHz, for example, may be obtained by adjusting the voltage of the power supply B. A signal voltage S is supplied to the input terminal IN. The oscillator VCO will generate at the output terminal OUT thereof an oscillatory signal whose frequency varies in response to the signal voltage S input to an input terminal IN of the electronic device and about the frequency (41.85 MHz) determined by the control voltage input to the control terminal C.
This exemplary volume controlled oscillator VCO employs a general purpose compact integrated circuit and has generally acceptable system characteristics under certain design conditions. However, it has a poor temperature characteristic in that the oscillation frequency tends to appreciably decreases as the ambient temperature increases, as shown in FIG. 11(b).
A conventional solution to eliminate such temperature dependence of the oscillator system includes use of a high-quality external resistor R having less temperature dependence or a special resistor which has a counter-temperature dependence to cancel out even the temperature characteristic of the oscillator VCO.
However, use of such a costly external resistor or seeking for a different semiconductor circuit, or even limiting the usable range of the oscillator VCO is by no means a favorable solution to the problem from the points of cost, manufacturing efficiency, and usability of the VCO.
It is therefore an object of the invention to provide a temperature compensation circuit utilizing ordinary electronic elements which enables an electronic device such as a voltage controlled oscillator (VCO) to maintain its required temperature characteristic.
In accordance with one aspect of the invention, the temperature compensation circuit for an electronic device comprises:
at least one semiconductor element having a PN junction adapted to receive on the P-layer thereof an input voltage;
an output circuit for regulating the voltage impressed across said PN junction (PN junction voltage) and adding said regulated PN junction voltage to said input voltage, thereby generating a control voltage for said electronic device canceling out the temperature characteristic of said electronic device.
This output circuit generates a control voltage which changes with ambient temperature to oppose the thermal change in the output of the electronic device caused by the ambient temperature change.
The output circuit may have a sufficient number of semiconductor elements, each having a PN junction, for generating a control voltage to cancel out said temperature characteristic of said electronic device.
This is based on the fact that the counteractive temperature effect of the semiconductor elements depends on the number of the elements.
The output circuit may have a first and a second resistors connected in series with each other and in parallel with said PN junction(s) such that said control voltage is available at the node of said first and second resistor.
In this arrangement, said first and second resistors may have resistances such that said control voltage cancel out the temperature characteristic of said electronic device.
In cases where one of the two resistors is chosen to have an infinitely large resistance, a necessary control voltage may be obtained to cancel out the temperature characteristic of the electronic device by simply adjusting the resistance of the other resistor.
In accordance with another aspect of the invention, the temperature compensation circuit comprises:
a bipolar transistor having a base for receiving thereon an input voltage;
a series third resistor connected between a power supply and said bipolar transistor;
a first resistor connected between said base of said bipolar transistor and the output end of said temperature compensation circuit; and
a second resistor connected between the node of said bipolar transistor and said third resistor and said output end.
It should be appreciated that this temperature compensation circuit has a simple structure that the temperature characteristic of the base emitter junction of the bipolar transistor is coupled with the temperature characteristics of the first and the second resistors. Nevertheless, the compensation circuit may provide a control voltage for desired temperature compensation to an electronic device such as a voltage controlled oscillator (VCO) or an electronic device utilizing such oscillator.
In this arrangement, one of the first and the second resistors may have an infinitely large resistance, in which case a preferable control voltage to cancel out the characteristic thermal change of the electronic device may be obtained by simply adjusting the resistance of the other resistor.
In accordance with a further aspect of the invention, the temperature compensation circuit comprises:
a bipolar transistor having a base for receiving thereon an input voltage (hereinafter referred to as input bipolar transistor);
at least one diode or diode-connected bipolar transistor connected in series with said input bipolar transistor;
a third resistor connected in series with said input bipolar transistor and said at least one diode or diode-connected bipolar transistor, said series connected between a power supply and a grounding line;
a first resistor connected between said base of said input bipolar transistor and an output end of said temperature compensation circuit; and
a second resistor connected between the node of said diode or diode-connected bipolar transistor and said output end.
In this simple arrangement, varied temperature compensation may be obtained for an arbitrary voltage controlled device, subject to an ambient temperature change, by combining the temperature characteristics of the first and the second resisters with the temperature characteristic of the multiplicity of series diode-connected bipolar transistors. The series bipolar transistors are equivalent to a diode having a large base-emitter voltage across the base-emitter junction. In fact, the temperature compensation circuit can substantially cancel out an over all temperature characteristic of a semiconductor IC device such as voltage controlled oscillator or of an electronic device utilizing a voltage controlled oscillator.
In this example also, one of the first and the second resistors may have an infinite resistance while the other may have zero resistance, and a preferable control voltage may be obtained to cancel out the temperature characteristic of the electronic device by simply adjusting the number of the series diodes or diode connected bipolar transistors.